How do MTU and jumbo frames work on a switch, and what breaks when they do not match?
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Say this first
The layer 2 MTU is the largest payload a port will carry, 1500 bytes by default. Jumbo frames raise it to about 9000 bytes for storage, vMotion and overlays. A switch drops oversized frames silently and only counts them, with no ICMP back, so a mismatch looks like small packets working while big transfers hang. Every layer 2 hop and both hosts must agree.
Go deeper
Layer 2 MTU and IP MTU
- Layer 2 MTU: largest frame payload a switch port accepts and forwards.
- IP MTU: largest IP packet a routed interface sends without fragmenting. A layer 3 switch has both:
a switching MTU and an
ip mtuon each SVI or routed port. - Only a router can fragment or send ICMP "fragmentation needed". A switch just drops.
Syntax differs a lot by platform
| Platform | Command |
|---|---|
| Catalyst 9000 | system mtu 9198 (global) |
| Catalyst 3560/3750 | system mtu jumbo 9000, then reload |
| Nexus 9000 | mtu 9216 per interface |
| Arista EOS | mtu 9214 per interface |
| Junos | set interfaces xe-0/0/1 mtu 9216 (counts the Ethernet header) |
| Linux | ip link set dev eth0 mtu 9000 |
What breaks
- Ping works, SSH works, file copies or iSCSI stall: the handshake is small, the data is not.
- VXLAN adds 50 bytes over IPv4, so the underlay needs at least 1550 for 1500-byte guest frames.
- On routed links an IP MTU mismatch also blocks OSPF adjacencies in EXSTART.
Test it properly
SW1# ping 10.0.0.2 size 9000 df-bitping -M do -s 8972 10.0.0.28972 = 9000 minus 20 (IP header) minus 8 (ICMP header).
They will ask next
What is a baby giant?
A frame slightly over 1518 bytes because of extra headers: 1522 with one 802.1Q tag, 1526 with QinQ, 4 more bytes per MPLS label. Most switches accept one tag by default; QinQ and MPLS need a small MTU increase.
Should you enable jumbo frames everywhere?
A common design is to set every switch to its maximum MTU, since a switch is just a pipe, and then choose the real MTU on the hosts per VLAN. All hosts in one VLAN must use the same value, because nothing in layer 2 can fragment between them.
Traps
- Testing with a plain ping. Without DF and a large size, the stack fragments and the test passes on a broken path.
- Mixing up Junos MTU, which includes the 14-byte Ethernet header, with Cisco MTU, which does not.